Apparatus known as sensitometers have been used for many years to expose samples of photographic papers and films in a very precise manner for subsequent densitometric analysis in which the density is measured of the image produced by such exposure. Such apparatus and the associated methods must be capable of exposing the photosensitive sample with a high degree of precision, accuracy and repeatability. More particularly, excellent control must be achieved for factors such as the illuminance at the exposure plane or the amount of radiant energy per unit area of the sample at a given point in time, the exposure time or the period during which the sample is exposed to radiant energy of such illuminance, the color temperature or spectral distribution of the radiant energy reaching the sample and the uniformity of the exposure across the surface of the sample. In the testing of photographic films and papers, a further concern is to be able to test the sample at illuminances, exposure times and color temperatures which closely approximate those to which end users subject the actual products. For testing purposes incident to manufacture of such films and papers, these three aspects of the exposure must be controlled more precisely than ever would be required by the end user. To maintain high productivity, some end users want to expose the product at faster and faster shutter times. To test a sample at such exposure times, the illuminance at the exposure plane must be increased in inverse proportion to the exposure time if the sample is to receive the same total amount of radiant energy as at a slower shutter speed. The flexibility to meet such changing test requirements has been largely absent from known sensitometric systems in which radiant energy requirements are high.
In known sensitometers, the radiant energy source typically is located far enough away from the sample that the sample is essentially uniformly illuminated and still receives enough radiant energy for proper exposure. Exposure times are created by a variety of shutter mechanisms, located either very close to the light source or very close to the sample. The radiant energy reaching the sample is attenuated through a test object or wedge or step tablet located between the source and the sample, with the sample usually pressed flat against the test object. Such test objects are made from a material transparent to the radiant energy to which have been added graded amounts of some spectrally neutral attenuating material, such as carbon or Inconel in the case of visible light, often in twenty-one individual steps measuring about 10 mm by 10 mm (0.39 by 0.39 in). Thus, radiant energy passing through the test object is attenuated by the added material before striking the sample. Often, the exposed and processed sample has an exposed area measuring about 210 mm by 10 mm (8.27 by 0.39 in) which is made up of twenty-one contiguous steps, each step in the test object being made to attenuate radiant energy differently than its neighbors. Such test objects typically attenuate visible light by 0.10 log, 0.15 log or 0.20 log increments to form what are called 0-2, 0-3 and 0-4 gradient test objects, respectively.
While such sensitometer apparatus and methods have long been used with acceptably good results, a variety of problems have been identified. Because of the way such test objects are made, it is difficult to set with precision the degree of attenuation in each step and it is not possible to change the attenuation in any step once the object has been made. For the same reason, it is difficult to make any two objects just alike. In addition, the exposure within a given step of such an object is occasionally found to be unacceptably non-uniform. The amount of radiant energy striking the test object and sample is limited by how close the light source can be placed to the sample and still uniformly illuminate the test object. The shuttering systems used in known apparatus and methods are limited in their ability to produce very short shutter times without sacrificing efficiency. For example, shorter exposure times are achievable if each step is exposed individually, which decreases precision and slows down the process. While different test objects can be used to provide different stages of exposure, each variant test object must be custom specified and fabricated, making it virtually impossible to expose a product in a non-conventional manner without an extended period of preparation. Such known test objects are easily broken or scratched and tend to accumulate dust which degrades the exposure. To maintain a desired color temperature, the voltage of the lamp is adjusted, which changes the illuminance at the exposure plane, making it necessary to move the sample closer to or further from the light source.
One prior art sensitometer known to the present inventors includes a logarithmically proportioned bundle of optic fibers to direct light from a source to several areas of a strip of sensitized material. The light source comprises a lamp, a spherical reflector and a condenser lens. An elliptical reflector also has been used without the condenser lens. A defocused image of the lamp filament is projected at the light receiving end of the optic fiber bundle. The light is spectrally modified with color balancing filters and an infrared filter. A shutter controls the exposure time of the light. Downstream of the light receiving end, the bundle is divided into a plurality of smaller fixed bundles of differing numbers of fibers; so that, each of the smaller bundles transmits a different amount of radiant energy. The number of fibers in each of the smaller bundles is such as to provide a given set of illuminance levels at an exposure plane. The exact illuminance level desired is achieved by correctly positioning a variable neutral density attentuator at the entrance to a light integrating chamber or column for each smaller bundle of fibers. Some of the smaller bundles also have a color filter adjacent their attenuator. The light output from each smaller bundle is directed through such attenuator or filter or both and into the integrating chamber. The fibers are attached at a fixed position near the entrance to the chamber and the light emitted from each smaller bundle is integrated through multiple reflections along the interior length of the chamber whose walls are painted white.